1. Field of the Invention
This invention relates to sonic transducers and more particularly to such transducers which include a piezoelectric crystal which is bonded to a diaphram.
2. Description of the Prior Art
The term "sonic transducer" as used herein refers to a device which may be mechanically deformed by application of an electrical signal thereto. The term also includes a device which generates an electrical signal in response to mechanical deformation. When a periodic voltage is applied to a sonic transducer, mechanical compression waves are generated therein which produce a standing wave pattern in the device. A sonic transducer is typically designed to operate at a selected frequency. It should be appreciated that the term as used herein includes devices which operate as described regardless of the operating frequency.
One application for a sonic transducer is as an ultrasound therapy device in the medical field. In such an application, a periodic voltage is applied to the sonic transducer which is placed against the skin of a patient undergoing treatment. Ultrasonic energy generated by the transducer heats up the tissue beneath the skin thereby producing beneficial therapeutic effects.
A sonic transducer or applicator in an ultrasound therapy device sometimes includes a substantially planar diaphram which is typically made of metal. A substantially planar piezoelectric crystal is bonded to the rear surface of the applicator which in turn is mounted on a handle held by an operator of the ultrasound therapy device during treatment. The handle is connected by an electrical cable to a periodic voltage source which is applied through the handle to the crystal. When the voltage source is energized, the crystal vibrates thus generating vibrations in the applicator which may be placed against the skin of a patient.
A piezoelectric crystal has only a very narrow range of frequencies in which it can vibrate with optimum deflection. Each crystal typically has a minimum impedance (known as the point of resonance) which occurs within one narrow frequency range and a maximum impedance (known as the point of anti-resonance) which occurs within a second narrow frequency range. The circuit may be designed to apply voltage either within the first range or the second range in order to optimize deflection.
When the crystal is vibrated in its optimum frequency range, crystal deformation is at a maximum and thus ultrasonic energy generated by the device is maximumized. Although the crystal may vibrate outside of this range, crystal deformation is substantially reduced thereby reducing the ultrasonic energy generated.
Sophisticated electronic circuitry has been developed to automatically tune the frequency of the voltage source to the point of resonance or anti-resonance, depending upon the circuitry configuration. A problem which exists with this scheme, with either automatic or manual tuning, relates to the fact that man-made crystals typically have a number of small impedance peaks and valleys as the frequency of the voltage applied to the crystal varies. Thus, it may be possible to unknowingly tune the voltage source to a frequency which is centered about one of the many small peaks or valleys which does not represent the absolute maximum, or minimum if tuning to the point of resonance, impedance.
In order to overcome the above-enumerated problems it would be desirable to produce a crystal which has a relatively broad range of frequencies in which it could be vibrated while generating maximum crystal deformation. It is known in the prior art that modifying the surface of a crystal to produce a substantially planar surface on one side and a substantially concave surface on an opposing side produces a crystal which operates at substantially its maximum energy output over a relatively broad range of frequencies. Since shaping crystals is a complicated and expensive process, this approach has not been universally accepted.